In this article, CAC to empower SUN inspiring from CR paradigm in wireless terrestrial communications is proposed. Spectrum scarcity in UAC due to the uniquely challenging underwater acoustic spectrum is discussed, and the need for spectrum-aware communication techniques is pointed out. We explore the capacity gain that can be achieved via CAC in SUN by simulation experiments and investigate the advantages and limitations of SUN along with its tradeoffs for DSA and OSA separately. Clearly, SUN can reach higher capacities than traditional fixed-spectrum approaches with the help of CAC capability. We expect that this article will provide better recognition for the capabilities of SUN and actuate further research efforts to explore this favorable area.